Kansas Geological Survey, Subsurface Geology 12, p. 43
Harold B. Rollins1, Ronald R. West2, and Richard M. Busch3
1University of Pittsburgh
2Kansas State University
3West Chester University
Application of computer simulation of genetic units in stratigraphic sequences requires meaningful field testing. Small-scale (i.e. 5th and 6th order: Busch and Rollins, 1984; Busch and West, 1987) genetic units may be either autogenic or allogenic. Autogenic genetic units are the depositional results of such events as delta switching, storm scour, or local tectonism and thus are not geographically widespread. Allogenic genetic units result from sea-level or climatic changes and therefore are correlative over wider geographic areas. The practical differentiation of autogenic and allogenic units is scale-dependent and has to be assessed prior to any specific stratigraphic analysis. Intrabasinal studies, for example, may define allogenic genetic units in terms of tectonostratigraphic dynamics constrained to a single basin, whereas these same genetic units might be autogenic in an interbasinal study concerned with global sea-level changes. However, in order to ascertain whether a unit is autogenic or allogenic, the stratigrapher must attempt to trace each genetic unit to its geographic limits.
The field testing of computer-modeled late Paleozoic sequences generally involves precise recognition of paleobathymetric changes, usually expressed as rather sharp contacts (genetic surfaces) of autogenic or allogenic origin. Typical allogenic surfaces are the results of either marine transgression or climate-change (Busch et al., 1989). Transgressive, surfaces are recognized where 1) a marine facies abruptly overlies a nonmarine facies, 2) a "more normal marine" facies abruptly overlies a "restricted" facies, or 3) a relatively deeper marine facies abruptly overlies a shallower facies. Determination of "more normal marine" and "relatively deeper" will often rely upon detailed assessment of faunal paleoenvironmental tolerances and paleobathymetry. Such surfaces may be cryptic in the interiors of depositional basins, perhaps discernible only as abrupt changes in time-averaged taxonomic diversity, extensive epibiont infestation, crevice faunas, abrupt biofacies boundaries, or condensed sections displaying complex shell beds. Transgressive surfaces are more easily recognizable, and typically less numerous, near basin margins where they may be associated with heterochronous deposition (exhumation and redeposition into time-averaged condensed sections), erosion or ravinemant, nondeposition (firmgrounds, hardgrounds, palimpsested surfaces, Trypanites and Glossifunqites ichnofacies), or any combination of the above. Climate-change surfaces may be indicated by 1) subaqueous-nonmarine facies abruptly overlying subaerial-nonmarine facies, 2) deeper subaqueous-nonmarine facies abruptly overlying shallower (relatively) subaqueous-nonmarine facies, 3) a relatively less restricted subaqueous-nonmarine facies abruptly overlying a relatively restricted subaqueous-nonmarine facies, and 4) a relatively more humid subaerial-nonmarine facies abruptly overlying a relatively less humid subaerial-nonmarine facies.
St. Catherines Island, in the shallow Georgia embayment, provides a modern analog for detailed field study of many basin-margin features noted in late Paleozoic stratigraphic sequences (West et al., in press; Morris and Rollins, 1977; Pemberton and Frey, 1985). The relict marsh muds exhumed along the seaward edge of the island record minor shoreline fluctuations (both allogenic and autogenic) in the form of palimpsested surfaces, Trypanites and Glossifunqites ichnofacies, transgressive shell lags, etc. Many of these features have analogs in the Pennsylvanian strata of the Appalachian basin and the midcontinent United States.
Busch, R. M., and Rollins, H. B., 1984, Correlation of Carboniferous strata using a hierarchy of transgressive-regressive units: Geology, v. 12, p. 471-474
Busch, R. M., and West, R. R., 1987, Hierarchal genetic stratigraphy--a framework for paleoceanography: Paleoceanography, v. 2, p. 141-164
Busch, R. M., Rollins, H. B., and West, R. R., 1989, Recognition of genetic surfaces and genetic units in late Paleozoic stratigraphic sequences; in, D. R. Boardman, J. E. Barrick, J. Cocke, and M. K. Nestell (eds.), Middle and Late Pennsylvanian Chronostratigraphic Boundaries in North-central Texas--Glacial-eustatic Events, Biostratigraphy, and Paleoecology, part H, contributed papers: Texas Tech University, Studies in Geology, v. 2, p. 325-331
Morris, R. W., and Rollins, H. B., 1977, Observations on intertidal organism associations of St. Catherines Island, Georgia--I. General description and paleoecological implications: Bulletin American Museum of Natural History, v. 149, no. 3, p. 87-128
Pemberton, S. G., and Frey, R. W., 1985, The Glossifunqites ichnofacies--modern examples from the Georgia coast U.S.A.; in, Biogenic Structures--Their Use in Interpreting Depositional Environments, H. A. Curran (ed.): Society of Economic Paleontologists and Mineralogists, Special Publication 35, p. 237-259
West, R. R., Rollins, H. B., and Busch, R. M., in press, Taphonomy and an interfidal palimpsest surface--implications for the fossil record: Paleontological Society, Special Publication
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Web version May 11, 2010. Original publication date 1989.